Wednesday, February 27, 2013

Answers to problems of 2-27-13.

Question:  I posted this problem this morning.

Two quick math problems before school today

17/20 + 10/25

17/20 - 10/25

One quick hint

What is the quickest way to find the common denominator?

I will post the answer on this web site in two hours.

I will also post the answer on

Answer:  When I checked my son's work I asked how did you get the common denominator of 100.  He said I just saw it.

Well sometimes you just won't see it because it won't be obvious.  You need a systematic method to find the least common denominator.

One way to find LCD is to start counting from the higher of the two numbers.  25, 50, 75, 100, 125.
Then see what number is divisible by 20.    Well 25, 50 and 75 are not divisible by 20 but 100 is.

So 17/20 is the same as 85/100.


Well we multiplied 20 by 5 so we had to multiply 17 by 5 and 17X5=85.

So 10/25 is the same as 40/100.   We multiplied 25 by 4 to get 100 and 10 by 4 to get 40.

The first problem 17/20+10/25 is the same as 85/100+40/100.

This equals 125/100 which is the same as 5/4 (divide the numerator and denominator by 25).  this is the same as 1 1/4 Divide 4 into 5.  Get  1 with a remainder 1.

(Note you can divide 100 into 125 and get a remainder of 25.  25/100 is equal to 1/4. Divide both the numerator and the denominator by 5.)

The subtraction problem:

17/20-10/25 is the same as 85/100 - 40/100 which is 45/100.  Divide the numerator and the denominator by 5 and get 9/20.

Is this right.  Always remember to check my work and your own work.  It is sometimes hard to check work because you remember what you did and think it correct.  Over time you will get better at checking.

I sometimes make errors.  If you see a problem either comment or email me at

Hope you enjoyed today's problems!!!

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